CSV file encoding not explicitly set to UTF-8

## Description

\FileProcessor.cs\ uses \
ew StreamWriter(filePath)\ which defaults to UTF-8 without BOM. However, the [OneRoster 1.1 Best Practice Guide (section 4.1)](https://www.imsglobal.org/oneroster-v11-final-best-practice-and-implementation-guide) requires:

> 'The CSV files must be UTF-8 encoded [RFC3629].'

While the default StreamWriter behavior produces UTF-8, being explicit avoids ambiguity across platforms.

## File to fix

\FileProcessor.cs\ line 25 — change to:

\\\csharp
using var writer = new StreamWriter(filePath, false, System.Text.Encoding.UTF8);
\\\

## Impact

Low — current output is already UTF-8, but explicit encoding removes any ambiguity.

Apri FileProcessor.cs alla riga 25 e controlla come viene creato StreamWriter. Rendi esplicita la codifica come UTF-8, quindi verifica che l'output CSV generato rimanga codificato in UTF-8 e che i controlli esistenti del progetto continuino a passare.

Scritto dal modello di indicizzazione a partire dal testo della issue.